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Temperature Sensors

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- Resistance Temperature Detectors (RTD) : consists of a small coil of platinum or nickel wire protected by a sheath of stainless steel. Its resistance increases with the temperature. The range of these transducers extends from 00 C to 6000 C (11120 F).
- R[T] = R[To] [ 1 + αo (T – To]
Where T is temperature at which the resistance is desired

To is the mid-point of valid temperature

R[T] is the resistance at T predicted

R[To] is the resistance at To from the table

αo is the fractional change in temperature per unit of temperature at To

The value α all metals will be on the order of 0.4% of the RTD resistance for every change of 10 C, in other words, the value is 0.004 / 0 C .

RTD has drawbacks:

Power dissipation factor

Self heating effect

Delay time constant

non-lineariarity

- Thermistor is a semiconductor based transducers. It has very good sensitivity. It is 10%. It is good for small temperature variations.
- Drawbacks:
Self heating effect

Time constant

Power Dissipation

Time Delay

- The thermocouple (TC) is a temperature sensor that produces a voltage directly as a function of temperature. The range is from
-150 0 C to 2000 0 C

- Interpolation
T(V) = T1 + [T2 - T1][V – V1]/[V - V1]

- There are types E, J, K, C, R, S
R and S are high temperature sensors used to measure temperature in the semiconductor furnace as the one in the Microelectronic Engineering clean room.

- Type Elements mv/0 C Range0C Error(%FS) Appliction
- EChromel/Constantan 0.044 0 to 800 0.5 High output
- JIron/constantan 0.033 -250 to 700 0.75 Reducing atm
- KChromel/Alumel 0.020 -250 to 1200 0.75 Oxidizing
- R & S Pt-Rh/Platinum 0.010 0 to 1400 0.25 Corrosive
- TCopper/Constantan 0.040 -250 to 350 1.00 Moist
- C Tungsten/rhenium 0.012 0 to 2000 0.5 High temp.

- Potentiometric: Its resistance varies as its wiper position changes.
Inexpensive

Physical contact wears out after long hours of use, generating noise as a result.

- LVDT is based on the electrical transformer
The difference of output voltages of two secondary winding changes according to the position of the core. AC signal is used, typically 1000 Hz signal is used as the source signal. Phase sensitive detector can be used to convert AC to DC output.

- Its resistance changes as the platinum wire changes its length and cross-sectional area.
GF = (δR/R)/(δL/L)

Where

GF is gage factor

δR/R is resistance ratio

δL/L fractional change in length, which is called the strain.